The present invention relates to a method and apparatus for controlling thermal printers, preferably to form binary images. More particularly, the present invention relates to a method and apparatus for selectively positioning boundaries between active and inactive areas of an image.
Thermal printing is generally accomplished by incrementally moving or "stepping" a print media in a y-direction relative to a stationary thermal print head. Thermal print heads generally include a plurality of heating elements which are frequently arranged in a linear array in an x-direction perpendicular to the direction of print media movement. The step size is often chosen to be equal to x-direction spacing or "pitch" between heating elements. For example, printers with 300 dot per inch (dpi) thermal print heads (i.e., one having a pitch of 1/300th of an inch) frequently step the print media past the thermal head at 1/300th of an inch intervals. The 1/300th.times.1/300th square inch area covered by a dot from a heating element is generally referred to as a picture element, or "pixel".
The heating elements are generally resistive elements each driven with a separate current flowing therethrough. A print controller selectively controls the respective heating element currents to activate the heating elements as desired during each advance of the print media. Different heating elements may be selectively activated for each step of the printing media, forming a two-dimensional image on print media.
The heating elements cooperate either directly to a thermally responsive printing media, or indirectly through donor media onto ordinary printing media. In the first case, referred to as "direct thermal printing", the printing media changes color in selective locations corresponding to the selectively energized heating elements. In the second case, referred to as "donor printing" or "thermal transfer priming", selective heated locations of the donor media transfer color to the printing media. The donor media may be a ribbon coated with a wax or ink which melts due to the heat of the heating elements.
There is a continual desire for thermal printers that provide clearer, more uniform and more consistent output images. Generally this desire has been addressed by reducing the spacing between the heating elements. However, the size of the heating elements and the distance between the heating elements is limited by present design and manufacturing considerations. Usually each heating element must be independently wired to a driver device which provides electrical drive energy to the heating element. As heating element spacing gets smaller, placement of these structures becomes more difficult and expensive. In addition, thermal printers should have relatively modest processor and memory requirements. In general, the greater the processor and memory requirements, the greater the cost of the thermal printing device.